Metal (hydro)oxides are among the most effective heterogeneous water oxidation catalysts. Elucidating the interactions between oxygen-bridged metal sites at a molecular level is essential for developing high-performing electrocatalysts. Here we demonstrate that adjacent metal-hydroxyl groups function as intramolecular proton–electron transfer relays to enhance water oxidation kinetics. We achieved this using a well-defined molecular platform with an aza-fused π-conjugated microporous polymer that coordinates molecular Ni or Ni–Fe sites that emulate the structure of the most active edge sites in Ni–Fe materials for studying the heterogeneous water oxidation mechanism. We combine experimental and computational results to reveal the origin of pH-dependent reaction kinetics for O–O bond formation. We find both the anions in solution and the adjacent Ni3+–OH site act as proton transfer relays, facilitating O–O bond formation and leading to pH-dependent water oxidation kinetics. This study provides significant insights into the critical role of electrolyte pH in water oxidation electrocatalysis and enhancement of water oxidation activity in Ni–Fe systems. (Figure presented.)

The conversion of CO<inf>2</inf> into value-added chemicals is highly critical for sustainable development. Among the various strategies, the dicarboxylation of alkenes with CO<inf>2</inf> offers a highly attractive route to access synthetically valuable dicarboxylic acids, which serve as key intermediates in the production of polymers and pharmaceuticals. Photoelectrochemical (PEC) carboxylation represents an efficient and sustainable carboxylation strategy, offering distinct advantages including mild reaction conditions, cost-effectiveness, and environmental compatibility. In this study, an efficient PEC system is presented for the carboxylation of styrene using a Ni-modified p-type micro-pyramid silicon (Ni/p-Si) photocathode. The incorporation of the Ni catalyst significantly suppresses charge recombination and accelerates charge transfer at the electrode-electrolyte interface, thereby enhancing the overall photoelectrochemical performance. The optimized Ni/p-Si photocathode achieved 77.7% faradaic efficiency (FE) for phenylsuccinic acid at −2.4 V vs. Ag/AgCl, with a photocurrent density of −4.5 mA cm⁻². Moreover, this PEC platform demonstrates moderate FEs across a range of substituted styrenes, indicating good functional group tolerance. Mechanistic studies reveal that the reaction proceeds via single-electron reduction of styrene to generate radical anions, which undergo CO<inf>2</inf> addition followed by further reduction and subsequent attack on a second CO<inf>2</inf> molecule to yield succinic acid. These findings broaden the scope of CO<inf>2</inf> utilization through selective and sustainable C-C bond formation processes.

A water oxidation catalyst Ru-bcs (bcs = 2,2′-bipyridine-6′-carboxylate-6-sulfonate) with a hybrid ligand was reported. Ru-bcs utilizes the electron-donating properties of carboxylate ligands and the on-demand coordination feature of sulfonate ligands to enable a low onset potential of 1.21 V vs. NHE and a high TOF over 1000 s⁻¹ at pH 7. The adaptive chemistry uncovered in this work provides new perspectives for developing molecular catalysts with high efficiency under low driving forces.

The cobalt-coordinated polyterthiophene/hematite hybrid photoanode, denoted as Co@PTTh-N/α-Fe2O3, was fabricated to facilitate the photo-driven water oxidation reaction. The optimally aligned energy levels of the PTTh-N/α-Fe2O3 heterojunction promoted charge transfer and decreased charge recombination. The phenanthroline functional groups within PTTh-N coordinated with Co2+ ions that serve as effective catalytic centers for water oxidation, resulting in a significant enhancement of photocurrent.

Despite considerable research efforts on photoelectrochemical water splitting over the past decades, practical application faces challenges by the absence of efficient, stable, and scalable photoelectrodes. Herein, we report a metal-halide perovskite-based photoanode for photoelectrochemical water oxidation. With a planar structure using mesoporous carbon as a hole-conducting layer, the precious metal-free FAPbBr3 photovoltaic device achieves 9.2% solar-to-electrical power conversion efficiency and 1.4 V open-circuit voltage. The photovoltaic architecture successfully applies to build a monolithic photoanode with the FAPbBr3 absorber, carbon/graphite conductive protection layers, and NiFe catalyst layers for water oxidation. The photoanode delivers ultralow onset potential below 0 V versus the reversible hydrogen electrode and high applied bias photon-to-current efficiency of 8.5%. Stable operation exceeding 100 h under solar illumination by applying ultraviolet-filter protection. The photothermal investigation verifies the performance boost in perovskite photoanode by photothermal effect. This study is significant in guiding the development of photovoltaic material-based photoelectrodes for solar fuel applications.

The key challenge within artificial photosynthesis is achieving efficient electro- or photo-driven water oxidation catalysis, a necessary process to supply the protons for the reduction reactions, thereby enabling solar fuel production. To facilitate efficient water (photo)electrolysis for solar fuel production, this thesis focuses on two aspects: 1) elucidating the O-O bond formation mechanism and developing efficient, stable, and economical water oxidation catalysts (WOCs); 2) exploring stable, low-cost, light-absorbing photoanode materials that have suitable band structures and excellent charge diffusion properties.

Chapter 1 provides an overview of the development of homogeneous and heterogeneous WOCs, with a particular emphasis on the catalytic mechanisms. Subsequently, it introduces the advancements in light-harvesting materials for photoelectrochemical cells and highlights the progress in the burgeoning field of lead halide perovskite-based photoanodes.

Chapter 2 clarifies the physical and electrochemical characterization methodologies, along with the protocols employed for mechanistic investigations in this thesis.

Chapter 3 introduces a host-guest complex, self-assembled through Co²⁺ and cucurbit[5]uril (CB[5]), as a supramolecular WOC. This catalyst, Co@CB[5], was immobilized on indium tin oxide substrate and BiVO₄ photoanode for electrochemical and photoelectrochemical water oxidation. The role of Co@CB[5] in interfacial charge transfer is investigated by spectroscopic and electrochemical studies.

Chapter 4 reports a molecularly well-defined heterogeneous WOC with aza-fused, π-conjugated microporous polymer coordinated single cobalt sites (Aza-CMP-Co). Integrating experimental and theoretical results, this work highlights the significance of electrolyte pH and the role of regulating the intramolecular hydroxyl nucleophilic attack pathway in enhancing water oxidation activity.

Chapter 5 presents a stable formamidinium lead bromide (FAPbBr₃) photoanode for water oxidation to achieve an exceptionally low onset potential. Theoretical calculations and spectroscopic characterizations reveal the origin of low onset potential, which offers pivotal insights in guiding the development of photovoltaic material-based photoelectrodes for solar fuel applications.

Den huvudsakliga utmaningen inom artificiell fotosyntes är att uppnå effektiv elektro- eller fotodriven vattenoxidationskatalys, en nödvändig process för att tillhandahålla protoner för reduktionsreaktioner, vilket möjliggör produktion av solbränsle. För att underlätta effektiv vatten(foto)elektrolys för produktion av solbränsle fokuserar denna avhandling på två aspekter: 1) klargörande av O-O-bindningens bildningsmekanism och utveckling av effektiva, stabila och ekonomiska vattenoxidationskatalysatorer (WOCs); 2) utforskning av stabila, lågkostnads, ljusabsorberande fotoanodematerial som har lämpliga bandstrukturer och utmärkta egenskaper för laddningsdiffusion.

Kapitel 1 ger en översikt över utvecklingen av homogena och heterogena WOCs, med särskilt fokus på katalytiska mekanismer. Därefter introduceras framstegen inom ljusinsamlingsmaterial för fotoelektrokemiska celler och framstegen inom det växande området av blyhalid-perovskitbaserade fotoanoder lyfts fram.

Kapitel 2 klargör de fysiska och elektrokemiska karakteriseringsmetoderna, tillsammans med de protokoll som används för mekanismundersökningar i denna avhandling.

Kapitel 3 introducerar ett värd-gäst-komplex, självmonterat genom Co²⁺ och cucurbit[5]uril (CB[5]), som en supramolekylär WOC. Denna katalysator, Co@CB[5], immobiliserades på substrat av indiumtennoxid och BiVO₄-fotoanoder för elektrokemisk och fotoelektrokemisk vattenoxidation. Rollen för Co@CB[5] i gränsskiktets laddningstransfer undersöks genom spektroskopisk och elektrokemiska studier.

Kapitel 4 redogör för en molekylärt väldefinierad heterogen WOC med kvävesammansatta, π-konjugerade mikroporösa polymerkoordinerade enskilda koboltställen (Aza-CMP-Co). Genom att integrera experimentella och teoretiska resultat belyser detta arbete betydelsen av elektrolytens pH och rollen av att reglera den intramolekylära hydroxyl-nukleofila anfallsvägen för att förbättra vattenoxidationsaktiviteten.

Kapitel 5 presenterar en stabil fotoanod av formamidinium blybromid (FAPbBr₃) för vattenoxidation för att uppnå en exceptionellt låg startpotential. Teoretiska beräkningar och spektroskopisk karakteriseringar avslöjar ursprunget till den låga startpotentialen, vilket ger avgörande insikter för den fortsatta utvecklingen av fotovoltaiska materialbaserade fotoelektroder för solbränsletillämpningar.

High band gap FAPbBr3 perovskite solar cells have attracted tremendous interest in recent years due to the high open circuit voltage and good stability. Commonly a two-step method is used to prepare the FAPbBr3 perovskite film. Here a mixed solvent approach for the second step is introduced. Formamidinium bromide (FABr) in 2-propanol and methanol mixture was applied in the second step, which resulted in favorable properties such as suitable solubility, high-quality crystallization, large grain size, improved charge extraction properties, and suppressed non-radiative recombination processes, and further enhance the power conversion efficiency (PCE) from 4.06 to 7.87%. As previously reported, methylammonium chloride (MACl) can help to improve the morphology and crystallinity of perovskite. To further prove the versatility of such a mixed solvent strategy and enhance the photovoltage performance, a small amount of MACl was added to the FABr solution with mixed solvents, and a high PCE of 9.23% was achieved under ambient conditions.

Adequate hole mobility is the prerequisite for dopant-free polymeric hole-transport materials (HTMs). Constraining the configurational variation of polymer chains to afford a rigid and planar backbone can reduce unfavorable reorganization energy and improve hole mobility. Herein, a noncovalent conformational locking via S–O secondary interaction is exploited in a phenanthrocarbazole (PC) based polymeric HTM, PC6, to fix the molecular geometry and significantly reduce reorganization energy. Systematic studies on structurally explicit repeats to targeted polymers reveals that the broad and planar backbone of PC remarkably enhances π–π stacking of adjacent polymers, facilitating intermolecular charge transfer greatly. The inserted “Lewis soft” oxygen atoms passivate the trap sites efficiently at the perovskite/HTM interface and further suppress interfacial recombination. Consequently, a PSC employing PC6 as a dopant-free HTM offers an excellent power conversion efficiency of 22.2 % and significantly improved longevity, rendering it as one of the best PSCs based on dopant-free HTMs. 

Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, pi-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.